Secondary batteries are often used as power supplies for driving portable electronic appliances or the like for the purpose of economy and resource saving and their use is remarkably expanding in these years. In accordance with size reduction and performance up-grade of electronic appliances, the batteries loaded therein are required to have a small size, light weight, and high capacity.
While lead-acid batteries and nickel-cadmium batteries are conventionally used as secondary batteries, non-aqueous lithium secondary batteries featuring a high energy density have been recently proposed and used in practice. Lithium secondary batteries using non-aqueous electrolytes, however, have the problem that they have a low current density as compared with conventional secondary batteries using aqueous solution because the electrolyte using a non-aqueous solvent as an ion-conducting medium has a lower ion-conducting rate.
Attempts were made to overcome such problems, for example, by increasing the surface area of electrodes to increase their contact area with the electrolyte. More particularly, an electrode coating composition containing an active material and a polymer binder is applied to a current collector in the form of a thin metal foil to form a thin electrode layer thereon and a plurality of such coated foils are placed one on another or spirally wound with a separator interleaved therebetween. For example, Japanese Patent Application Kokai (JP-A) No. 121260/1988 describes a lithium secondary cell using a nonaqueous electrolyte solution, LiCoO.sub.2 and/or LiNiO.sub.2 as a positive electrode, and carbon as a negative electrode. However, some problems arise when an electrode layer is formed on a current collector typically in the form of a metal foil. Repetition of charge-discharge cycles exacerbates the interfacial adhesion between the current collector and the electrode layer and lowers the discharge capacity of the electrodes, resulting in a short cycle life. Fine particles of the electrode layer shed from the current collector can cause short-circuits.
One of the probable causes is that as the active material and electrode layer are expanded and contracted by doping and dedoping of lithium ions upon charging and discharging, local shear stresses are developed at the electrode layer-current collector interface, exacerbating the interfacial adhesion. As a consequence, the internal resistance of the battery is increased to invite a capacity lowering and an interior temperature rise, resulting in a short battery life.
Proposals were made to prevent deterioration of the bond between an electrode layer and a current collector for increasing current collecting efficiency. For example, Japanese Patent Publication (JP-B) No. 186465/1987, JP-A 148654/1990, and JP-A 238770/1991 disclose non-aqueous electrolyte cells wherein an electrode layer and a current collector are separated by a conductive underlying layer containing silica as a binder and carbon black or nickel fine powder. These underlying layers are effective when applied to primary cells. Problems arise in secondary cells where charge/discharge cycles are repeated. Since the underlying layer using silica as a binder is less flexible, it fails to accommodate shear stresses developed at the electrode layer-current collector interface as a result of expansion and contraction of the electrode layer during charge/discharge cycles. This underlying layer fails to prevent deterioration of the bond between the electrode layer and the current collector after repeated charge/discharge cycles.
JP-A 266774/1988 discloses a flat contour organic electrolyte cell including a current collector in the form of an ethylenic resin film containing carbon black. A layer containing a polymer which is more flexible than the current collector and carbon black is formed on the current collector. This cell has the problem that upon repeated charge/discharge cycles, the binder polymer is degraded and leached into the organic electrolyte, losing adhesion.
Also JP-A 91719/1975 discloses a current collector having an underlying layer formed of a thermoset composition comprising chlorosulfonated polyethylene, tribasic lead maleate, and carbon black. Differently stated, the technique of using a polymer as a binder in an underlying layer and thermosetting the polymer is disclosed. However, the polymer used in the underlying layer is chlorosulfonated polyethylene, which cannot achieve satisfactory properties after thermosetting because of inclusion of chlorine. There remains a problem that as charge/discharge cycles are repeated, the binder is degraded, resulting in a substantial loss of discharge capacity.